1. (a) Field of the Invention
The present invention relates to a revolution indicator which is suitable for use in measuring distances, particularly, for use in measurement of moving distances or positions of movable elements such as a table of a machine tool or the like.
2. (b) Description of the Prior Art
It is well known such a distance-measuring device that it includes a plurality of character-displaying wheels adapted to be rotated by rotational motions which are converted from linear motions of various mechanical elements. Such device is widely utilized at present due to its reliable operation.
On the other hand, the development of the automatic control technique provides a method of controlling the operations of various machines in various producing steps which is widely used nowadays. An encoder for such a purpose and an apparatus for indicating detection values by electronic means are provided.
Although the above electronic indicators are suitable for automatic control of the apparatus, there is a problem such that they don't operate in case of a power failure. When a power failure occurs, the recorded values just before the occurrence of the power failure are lost. In the machine tool or the like, therefore, it is dangerous to rely on only those electronic indicators. In many cases, accordingly, the mechanical revolution indicator is still often used.
On the basis of such circumstances, in JP-U-1-65839, the same inventor as the present inventor has already disclosed an electro-mechanical counter in which a rotary encoder is attached to a mechanical revolution indicator and an output of the encoder is counted by an electronic counter.
The encoder used ill such an apparatus, however, is of the incremental type and its output pulse is counted by the electronic indicator. When a power failure occurs, therefore, the recorded value of the electronic indicator returns to zero and thus differs from the indication value of the mechanical revolution indicator.
Therefore, in the case where such an apparatus is used as an apparatus for indicating the displaced position of a mechanical element from a reference point, it is necessary that such a mechanical element is once returned to the reference point and the recorded value of tile counter is reset and the indicated values of two indicators are made to coincide with each other.
Even when the operation of the machine is restarted after the stop of the operation or when the indication values of the mechanical revolution indicator and of the electronic indicator differ to each other, the adjustment as mentioned above are necessary.